Place lining inside of hat, wrong side facing the inside of the hat. Overlap open section to fit hat at the head size. One size lining should fit most hats. Always position seam in the back of the hats. Pin into hat starting at head size and checking the length from tip to head size after pinning around, trim edge at bottom of inside of hat. Use a 1/2 inch diagonal basting stitch to sew lining to hat. Make sure stitches are smaller than the sweatband that NEXT you will sew in (using the pick stitch) to cover the stitches and protect the lining from sweat, makeup, etc. Sweat band will be attached to the hat at the bottom of the crown ( see my Grosgrain book). The upper section will be left unattached. This means one could peek under the sweatband and see your “diagonal but parallel lines, all the exact size, perfectly proving that attention to detail makes your hat worth more $”
For those who know the difference!